1. Field of the Invention
The present invention relates to a turbo vacuum pump for evacuating a gas, and more particularly to a turbo vacuum pump suitable for evacuating a corrosive process gas or evacuating a gas containing reaction products. The present invention also relates to a semiconductor manufacturing apparatus having such a turbo vacuum pump.
2. Description of the Related Art
FIG. 16 of the accompanying drawings shows a conventional turbo vacuum pump disclosed in Japanese Patent Publication No. 2680156. As shown in FIG. 16, the conventional turbo vacuum pump comprises a casing 11 having an intake port 11A and-an exhaust port 11B, a rotating shaft 12 provided in the casing 11 and rotatably supported by bearings 16, and a centrifugal compression pumping section 13 and a peripheral compression pumping section 14 arranged successively in the casing 11 from the intake port side (the side of the intake port 11A) to the exhaust port side (the side of the exhaust port 11B). The centrifugal compression pumping section 13 comprises open impellers 13A fixed to the rotating shaft 12 and stationary circular disks 13B which are alternately disposed in an axial direction of the pump. The peripheral compression pumping section 14 comprises impellers 14A fixed to the rotating shaft 12 and stationary circular disks 14B which are alternately disposed in the axial direction of the pump. The rotating shaft 12 is rotated by a motor 15 coupled to the rotating shaft 12.
In the case where a corrosive gas is evacuated by the conventional turbo vacuum pump shown in FIG. 16, the casing 11, the rotating shaft 12, and the pumping sections 13 and 14 are required to have corrosion resistance. Further, in the case where a gas containing reaction products is evacuated by the conventional turbo vacuum pump, in order to prevent the reaction products from being deposited in the pumping sections 13 and 14, it is necessary to keep an evacuation passage at a high temperature. Therefore, it is desirable that the casing 11, the rotating shaft 12 and the pumping sections 13 and 14 are composed of materials having corrosion resistance and low coefficient of thermal expansion so that dimensional change caused by temperature change is small. Further, if the rotating shaft 12 is composed of a material having high strength and high Young's modulus, then high-speed rotation of the rotating shaft 12 can be easily achieved to enhance evacuation performance of the vacuum pump. Furthermore, it is desirable that the rotating shaft 12 is composed of a ferromagnetic material to improve output characteristics of the motor 15.
However, because very few materials have the characteristics of corrosion resistance, low coefficient of thermal expansion, high strength, high Young's modulus, and ferromagnetism all together, materials for the rotating shaft 12 must be chosen depending on its use or at the sacrifice of any of the characteristics. For example, as a material used frequently for the rotating shaft, there is Fe—Ni alloy such as Niresist cast iron. The characteristics of Fe—Ni alloy are corrosion resistance, low coefficient of thermal expansion, and ferromagnetism, but the Young's modulus of the Fe—Ni alloy is about 130 GPa and is smaller than that of a general steel material which is 206 GPa. Therefore, the critical speed of the rotor becomes low, and hence it is difficult to achieve high-speed rotation of the rotor. Thus, the rotational speed of the rotor is made lower at the sacrifice of evacuation performance of the vacuum pump. Alternatively, the diameter of the rotating shaft is made larger to achieve high-speed rotation of the rotor, thus failing to make the pump small-sized and lightweight.
Next, an example of a conventional semiconductor manufacturing apparatus which incorporates a vacuum pump will be described with reference to FIG. 17. As shown in FIG. 17, in a conventional semiconductor manufacturing apparatus 81, a vacuum evacuation system is constructed by a vacuum pump 83 provided outside of the apparatus and a piping 84 connecting a vacuum chamber 82 to the vacuum pump 83. However, in the case where a large amount of gas is flowed during a manufacturing process, or a pressure in the vacuum chamber is lowered, this construction frequently causes a problem of conductance of the piping 84. In order to solve this problem, the diameter of the piping 84 is made larger and the size of the vacuum pump 83 is made larger, thus increasing an initial cost and enlarging an installation space.
Further, a conductance variable valve 85 is provided in the piping 84, and the opening degree of the conductance variable valve 85 is adjusted so that the pressure of the vacuum chamber 82 is set to a desired value during a manufacturing process. However, the installation of the conductance variable valve 85 causes a lowering of the conductance and complicates the vacuum evacuation system.
FIG. 18 is a schematic view showing a support structure of a rotor in a conventional turbo vacuum pump. As shown in FIG. 18, the turbo vacuum pump comprises a rotor 303 having a stacked and multistage structure. In this vacuum pump, in order to make rotor blades 301 multistage, a hole 304 is formed in a central part of each rotor blade 301, and a rotating shaft 305 is inserted into the hole 304 of each rotor blade 301, whereby the rotor blades 301 are joined together.
However, in the case where the rotating shaft 305 is inserted into the holes 304 of the respective rotor blades 301, a motor 307 is attached to the rotating shaft 305, and a section including the rotor blades 301 and a section including the motor 307 are separated from each other, bearings 306 are disposed in the section including the motor 307. Therefore, the motor 307 is disposed between the bearings 306, and the rotor blades 301 are disposed outwardly of the bearing 306 located near the rotor blades 301, and hence the rotor 303 having the rotating shaft 305 and the rotor blades 301 is supported in such a state that the rotor blades 301 are overhung. That is, the rotor 303 becomes a cantilever structure. Therefore, natural frequency of the rotor 303 is likely to be lowered, and in some cases, it is difficult to achieve high-speed rotation of the rotor 303. Further, because a large load is applied onto the bearing 306 disposed near the rotor blades 301, this bearing 306 is required to be large-sized, resulting in a large-sized pump and an increase of vibrations.
Further, if an increase in evacuation capacity of the vacuum pump makes the rotor blades 301 larger in size and number, then the degree of the overhanging state of the rotor becomes larger to make the above situation worse. Consequently, in order to make the distribution of mass and rigidity appropriate, the rotating shaft 305 is required to be larger in diameter and length, or a balance weight is required to be installed, thus making the vacuum pump larger in size and weight.